Trichomonadida

Based mainly on:

Kulda, J., Nohýnková, E., Ludvík, J. (1986). Basic Structure and Function of the Trichomonad Cell. Acta Univesitatis Carolinae-Biologica. 30, 181-198

Trichomonadida together with Hypermastigida belong to phylum Parabasala.

Order Trichomonadida comprises families Monocercomonadidae (Monocercomonas, Hexamastix, Histomonas, Dienthamoeba), Trichomonadidae (Trichomonas, Tritrichomonas, Tetratrichomonas, Pentatrichomonas, Pentatrichomonoides), Calonymphidae (Calonympha) and Devescovinidae (Devescovina, Foaina, Metadevescovina). Trichomonadida are mostly harmless comensals of digestive tract of man and many other vertebrate and invertebrate hosts. Pathogenic species have often extraintestinal localisation. As pathogenic are regarded species Trichomonas vaginalis, Dienthamoeba fragilis, Tritrichomonas foetus, Histomonas meleagridis and Trichomonas gallinae. The evidence of pathogenity of Tetratrichomonas gallinarum is controversial. . Trichomonas vaginalis is causative agent of serious urogenital infection of man (human trichomonosis). Tritrichomonas foetus infects urogenital tract of cattle (bovine trichomonosis). Histomonas meleagridis cause often fatal infection of turkey and some other avian hosts.

Trichomonadida are amitochondrial eucaryotic cells equipped with characteristic mastigont. This assembly of cytoskeletal and membranous structures connected to kinetosomes includes, in addition to flagella, microtubular constituent of skeleton (axostyle and pelta), large striated root fibrils (costa and parabasal fibrils), unusual Golgi complex integrated with fibrils (parabasal apparatus) and variety of kinetosome associated lamellae, rootlets and filaments. Typical cytoplasmic organelle of trichomonads is the hydrogenosome, microbody-like redox organelle involved in energy metabolism of cell. Trichomonads lack structurally differentiated organelles of feeding comparable to cytostomes and feed by pinocytosis or both pinocytosis and phagocytosis. The cytoplasm contains large secondary lysosomes (digestive vacuoles) and coated vesicles; primary lysosomes were identified by EM cytochemistry. Rough endoplasmic reticulum is primarily located in the perinuclear area and around the axostyle. Cytoplasm is rich in free ribosomes. Glycogen, the main energetic store of trichomonad is present in the cytoplasm in the form of numerous alfa-rosettes. The nucleus of trichomonads possesses probably 6 chromosomes and divides by a specific type of mitosis with extranuclear spindle and retained nuclear membrane. The cell divides by binary fission. Round forms with multiple karyomatigonts result from defective cytokinesis and do not represent an integral life cycle stage. Trichomonads react to hostile conditions by internalising the flagella and the undulating membrane to form "pseuocysts". These can revert to normal trophozoites but do not present a resistant stage. The occurrence of true cysts with external cell wall is limited to a few species and cysts have not bee found in any pathogenic trichomonad.

Fig. Schematic diagram of trichomonad cell. Anterior flagella (AFl), kinetosomes (K), pelta (P), costa (C), nucleus (N), parabasal apparatus (G), parabasal fibril (PF), hydrogenosome (H), secondary lysosome (Ly), rough endoplasmatic reticulum (ER), axostyle (Ax), undulating membrane (UM), recurrent flagellum (RFl). (Drawing reprinted from Kulda, J., Nohýnková, E., Ludvík, J. (1986). Basic Structure and Function of the Trichomonad Cell. Acta Univesitatis Carolinae- Biologica 30, 181-198.)